Global–local processing and the Ebbinghaus illusion: Group and individual differences in young and older adults

The Ebbinghaus illusion is a size illusion, in which a central circle appears larger or smaller depending on the size of surrounding circles. This illusion is widely used to study group-level differences in attentional processing, with the proposal that a local-processing bias reduces susceptibility to the Ebbinghaus illusion. One hundred and forty-five participants (87 young; 58 older) were included in the analyses. Participants completed the Navon hierarchical-figures task, to measure the global–local processing bias, and the Ebbinghaus illusion task, to measure susceptibility to the illusion. First, we investigated whether a strong local-processing bias reduced susceptibility to the Ebbinghaus illusion. Our findings did not support this proposal. At the group level, older participants demonstrated worse performance for global processing compared to young participants, but there were no age-group differences in susceptibility to the illusion. At the individual level, the young and older participants with the stronger local-processing bias were the participants with greater susceptibility to the illusion. Second, we investigated whether longer inspection times during the Ebbinghaus illusion task reduced susceptibility to the illusion. Our findings did support this proposal. At the group level, there were no age-group differences in either inspection time or susceptibility to the Ebbinghaus illusion. At the individual level, we replicated previous findings—the participants with the longer inspection times were the participants with the least susceptibility to the illusion. We discuss alternative cognitive mechanisms that may account for the Ebbinghaus illusion and their relevance to age-related changes and individual differences in visual attention.